Susbtitution of alcohols

General Reaction for 3° alcohols:

General Reaction for 1° and 2° alcohols need ZnX₂ as a catalyst and heat:

The reaction of 1° and 2° alcohols is sluggish because the carbocation formed from the S_N1 mechanism is not stable enough, so this reaction must go through the S_N2 mechanism. The S_N2 mechanism requires the direct substitution of the nucleophile with the alcohol+catalyst complex to give the halide. Heat is required to make the reaction proceed.